This invention relates to optical spectroscopy, and particularly to focusing accessories for use in directing the radiation to, and receiving the radiation from, a sample under analysis.
Such accessories may conveniently be considered in two general categories. One category includes reflectance accessories in which the radiation leaving the sample holder is travelling in a different direction from the radiation reaching the sample holder because of its reflectance by the sample. The other category includes accessories in which the radiation leaving the sample holder is travelling in essentially the same direction as the radiation reaching the sample holder. In Doyle application Ser. No. 291,402, filed Aug. 10, 1981, and having the same assignee as this application, an accessory of the latter category is disclosed.
The present invention relates to accessories of the former category, and particularly to apparatus which is useful in analyzing samples both in the diffuse reflectance mode and in the specular reflectance mode. Although reflectance accessories of the type disclosed herein can be used with a wide variety of spectrometers, they are particularly suited for use with Fourier Transform infrared (FTIR) instruments and make use of the fact that the sample region beams in these instruments can be made available in a collimated form with a circular cross section.
Optical spectrometers normally function by transmitting a beam of radiation through a sample of a material under study. The wavelengths of the radiation beam are encoded in various ways so as to make possible the recording of a transmission spectrum of the material (ie: optical transmission vs wavelength). In all of the earlier instruments, and a majority of present instruments, wavelength encoding has been accomplished by using a diffraction grating or a prism to spectrally disperse the radiation. The radiation is then brought to a focus, allowing a slit to be used to select a narrow region of the spectrum for transmission through the sample, which is placed in the "sample" region immediately after the slit. The divergent nature of the radiation beam emerging from the slit establishes the constraints on sample size and shape as well as the design of sampling accessories.
Over the years, a large number of accessories have been designed for use with dispersive spectrometers. These allow the normal transmission geometry to be converted for use in such measurements as microsampling, attenuated total reflectance, specular reflectance and diffuse reflectance. In each case, the accessory has had to start with a diverging beam, refocus it at the surface of a sample, collect the transmitted or reflected radiation, and reconfigure it to appear as if it were still diverging from the original slit position. These multiple requirements have generally led to rather complex designs entailing typically five or six reflections, critical positioning in the sample compartment, and critical adjustment of the various mirror positions. Sheets 12-15 of the loose-leaf, undated catalogue of Harrick Scientific Corporation illustrate some of the typical designs.
Considerable difficulty with such accessory designs results from the need to start with a beam focused at a slit and end with a beam which appears to be diverging from the same slit position. Alteration of this basic geometry would require corresponding changes in the detector optics to enable the radiation to be collected and imaged on the optical detector.
Over the past ten years or so, Fourier Transform Infrared (FTIR) spectrometers have come into common use. In these instruments, wavelength encoding is accomplished by a Michelson interferometer rather than a dispersive device. One result is the elimination of the usual slit. In fact, the beam emerging from the interferometer is most typically collimated (to within a degree or two) and has a circular cross section. Despite this, FTIR manufacturers equip their instruments with auxiliary optics to bring the beam to a focus in the sample region. This is done to minimize required sample size and to make the instruments compatible with the large number of accessories designed for dispersive instruments. The sample region geometry of the Nicolet MX-1 spectrometer is illustrated in the Nicolet publication "Optical Layouts and Specifications of Nicolet FTIR spectrometers", revised March 1980.
Recently there has been considerable interest in diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and one FTIR manufacturer (Digilab) is selling an accessory specifically designed for this work, as illustrated in the paper by K. Krishnan, et. al in American Laboratory, March 1980, page 104. This device is designed for use with a focused beam and uses a total of five mirrors.
Fuller and Griffiths, as explained in an article in American Laboratory, October 1978, Page 69, have developed a diffuse reflectance accessory which maximizes the total signal reaching the detector, in part by dispensing with the normal sample region focusing optics. However, this design requires remounting the IR detector in a fixed position relative to the accessory and, therefore, sacrifices the convenience which results when an accessory can be simply dropped into the sample compartment without upsetting the FTIR instrument's detector alignment.
The present invention is intended to provide a reflectance apparatus, for use in spectroscopic instruments, which will have the following advantages over previous apparatus:
(1) Improved adaptability, and accurate adjustability, to permit use either for analysis of diffuse reflectance, or for analysis of specular reflectance; PA0 (2) Capability of being "dropped into" an existing spectrometric instrument without interfering with, or requiring repositioning of, the optical beam, the mirrors, or the detector, of the basic instrument; PA0 (3) Capability of locating the sample in a noninterfering position, while at the same time providing a small focal point size ahd high collection efficiency for micro-reflectance experiments; PA0 (4) Quality of aberration compensation because of preservation of the angular beam divergence, which is a significant advantage when used with specular samples; PA0 (5) Avoidance of any requirement for X, Y or Z translational adjustments of the accessory apparatus, and PA0 (6) Using a minimum number of optical elements.